This invention relates to switching the gamma (.gamma.) of shutter control time of a programmed shutter.
First, the gamma (.gamma.) will be described. In the APEX indication in photographic optics, the following equation is established. ##EQU1##
where E.sub.V is the exposure exponent; A.sub.V is the aperture value exponent; T.sub.V is the shutter speed exponent; S.sub.V is the film sensitivity exponent; and B.sub.V is the brightness exponent of an object.
The relation between the value T.sub.V and the actual time can be expressed as follows: EQU T=1/(2T.sub.V)
In general, in taking pictures it can be considered that the film sensitivity exponent S.sub.V is substantially constant for as long as the roll of film loaded in a camera is used. Therefore, a graph, shown in FIG. 1, can be obtained by plotting the values T.sub.V and E.sub.V respectively along the vertical and horizontal axes. If the quantity of unitary change of E.sub.V is designated by .DELTA.E.sub.V and the quantity of change of T.sub.V caused when the unitary change .DELTA.E.sub.V is made is designated by .DELTA.T.sub.V, then .DELTA.T.sub.V /.DELTA.E.sub.V is the gamma (.gamma.) of shutter control time. If the value S.sub.V is constant, the value E.sub.V is a function of the value B.sub.V.
Some programmed shutters serving as a shutter and a lens stop have a range (.gamma.&lt;1) where the operation is effectuated with a constant relation between time and aperture opening diameter (F value) when the brightness is relatively high, and a range (.gamma.=1) where the quantity of exposure is controlled only by the time with the aperture opening diameter maintained unchanged when the brightness is higher than a certain value. For such a programmed shutter, it is necessary to switch the aforementioned gamma (.gamma.) according to the brightness ranges.
In a conventional programmed shutter having the ranges .gamma.&lt;1 and .gamma.=1, a brightness detecting element made of cadium sulfide (hereinafter referred to merely as "a CdS" when applicable) is employed, and switching the gamma is accomplished by utilizing the fact that the resistance of the CdS provides (.gamma.&lt;1) in the case of the high brightness range and (.gamma..div.1) in the case of the low brightness range. Furthermore, the approximation of the gamma (.gamma.) is used as indicated in FIG. 2, in which reference numeral (1) designates a curve in which switching the gamma (.gamma.) is ideally performed, and reference numeral (2) designates a curve indicating the variations of resistance of the CdS.
However, the variation characteristic of resistance of the CdS with respect to a wide range of brightness is relatively inaccurate. Specifically, the variation characteristics of resistance of the CdS is not for example like the characteristics of a photo-diode, where the variations of optical current with respect to a wide range of brightness are of .gamma.=1. That is, with the CdS, the brightness range which is considered similar to the characteristics in which switching the gamma is ideally performed is limited, and accordingly the brightness range employable for a camera is, in general, limited. Furthermore, in general, it is difficult to switch the gamma only by means of the light receiving element. The response of the CdS to variations of light is slow, especially in the range of low brightness, and it is slow when compared with that of a photodiode. However, the variations of the output optical current of the photo-diode with respect to the entire range of brightness is .gamma.=1. Therefore, the photo-diode cannot be used for the programmed shutter without modification, which is controlled with the exposure control time for brightness being of .gamma.&lt;1.
Thus, an element such as a photo-diode whose output varies with .gamma.=1 in the entire range of brightness cannot be employed for a programmed shutter, especially a programmed shutter having ranges different in gamma (.gamma.), although the element is superior to the CdS.